Generally, a chemical mechanical polishing (CMP) apparatus, which utilizes a polishing pad and slurry to polish a wafer, is used for planarization of insulating material through wide area on a semiconductor wafer. The need for CMP is due to trends toward high integration and high speed semiconductor devices.
The wafer is attached to a lower surface of a carrier head capable of being rotated and horizontally fluctuated. The wafer is in contact with a rotating polishing pad and, therefore, the wafer is mechanically polished by its own weight, the weight of the carrier head, and applied pressure. The horizontal fluctuation of the wafer is defined as a straight reciprocation along a center and circumference of the polishing pad.
While the wafer is rotated on the carrier head, a polishing solution, i.e., a slurry, is supplied onto the polishing pad. The slurry flows into a fine gap between the wafer and the polishing pad. In other words, the polishing solution flows into pores of the polishing pad, thereby inducing a chemical reaction and thus chemically polishing the wafer.
As the polishing process progresses, the polishing rate of the wafer per unit hour is reduced. Accordingly, the polishing speed becomes slow. To prevent the reduction in polishing speed, a disc-type pad conditioner, to which a number of diamonds having a dimension of 100 to 200 μm is attached, is forcibly contacted with the upper surface of the polishing pad to recover a surface condition of the polishing pad. This process is referred to as pad conditioning.
A conventional CMP apparatus will now be described in conjunction with FIG. 1. The conventional CMP apparatus includes a carrier head 110, fixing a wafer 103 by absorption power. The carrier head 110 includes an adhesive carrier film (not shown) attached to a center of the lower surface of the carrier head 110. The wafer 103 is adhered and fixed to the lower surface of the carrier film and a ring-type retaining ring 111 is adhered to the lower surface of the carrier head 110 around the carrier film and wafer 103. The carrier head 110 is coupled to a driving means (not shown).
The retaining ring 111 serves to prevent the wafer 103 fixed to the carrier film from being detached from the carrier head 110 during the polishing process. In addition, the retaining ring 111 serves to displace vertical pressure acting on a center of the carrier head 110 onto a periphery of the carrier head 110.
A rotary turntable 101 is located at a position spaced apart from the lower portion of the carrier head 110, and a polishing pad 102 made of polyurethane is attached to an upper surface of the rotary turntable 101. The rotary turntable 101 is provided with a slurry-injecting nozzle 130 spaced apart from the upper surface of the rotary turntable 101 for supplying the slurry during the polishing process. A pad conditioner 120 is installed in a proper position above the polishing pad 102, independently of the carrier head 110.
When the polishing process of the wafer 103 is started, the rotary turntable 101 is rotated, with the polishing pad 102 being attached to the upper surface of the rotary turntable. The carrier head 110 is moved downwardly until the wafer 103 is in contact with the polishing pad 102. When the wafer is in contact with the polishing pad, the carrier head 110 is rotated and horizontally fluctuated by the driving means (not shown).
At the same time, the slurry is injected onto the polishing pad 102 from the slurry-injecting nozzle 130 to induce the chemical reaction between the slurry and the wafer, thereby achieving the chemical polishing in addition to the mechanical polishing.
After completing the polishing process for the wafer 103, or during performing the polishing process, the pad conditioning process is performed to increase polishing degree of the polishing pad 102. The pad conditioner 120 installed above the upper portion of the rotary turntable 101 is connected to a driving means, independently of the carrier head 110. Accordingly, the pad conditioner 120 is rotated to be forcibly in contact with the polishing pad 102, thereby recovering the surface condition of the polishing pad.
In the course of performing the CMP process, the wafer 103 has to be in contact with the polishing pad 102, and it necessarily requires mechanical action, such as rotation. Therefore, there is a problem of wafer scratching.
Explaining the actual process as an example, as shown in FIG. 4, an insulating layer 402 having a via hole 403 is formed on a semiconductor substrate 401. In this state, a metal layer 404 is deposited on the insulating layer 402 to sufficiently fill the via hole 403, the CMP process is performed to the metal layer to form a plug in the via hole 403. At that time, an end point (a point C of FIG. 4) of the planarization is a surface of the insulating layer. In other words, the surface of the insulating layer is planarized to form the plug in the via hole 403.
For performing the CMP process, the polishing pad of the CMP apparatus is closely in contact with the metal layer or insulating layer until the surface of the insulating layer 402 is exposed, thereby potentially forming scratches on the surface of the insulating layer.